1. Field of the Invention
The present invention relates to a semiconducting ceramic composition for secondary electron multipliers.
2. Description of the Prior Art
Up to now, there have been proposed various secondary electron multipliers employing a semiconductor ceramic material. For example, JP-B-48-18026 discloses a secondary electron multiplier comprising a cylindrical semiconducting ceramic body with electrodes provided thereon. JP-B-48-18029 discloses a secondary electron multiplier comprising a semiconducting ceramic plate having a pair of electrodes provided at opposed ends thereof and including several perforations formed therein perpendicular to both electrodes. JP-B-48-18030 discloses a secondary electron multiplier comprising a plurality of semiconducting ceramic tubes of a uniform length and bundled together.
Materials used in these electron multipliers are semiconducting ceramic compositions of a zinc titanate system. Examples are those consisting essentially of 72.5 mol % of ZnO and 27.5 mol % of TiO.sub.2, and those consisting essentially of 72.5 mol % of ZnO, 27.5 mol % of TiO.sub.2 and containing Al.sub.2 O.sub.3 incorporated therein in an amount of 1.25 mol %. The resistivity of these compositions are 8.times.10.sup.6 .OMEGA.cm for the former, and 2.8.times.10.sup.6 .OMEGA.cm for the latter, respectively. These semiconducting ceramic materials of the prior art have been applied to secondary electron multipliers used for the purpose of detecting a slight amount of charged particles.
However, the above semiconducting ceramic materials can not be applied to secondary electron multipliers to be used for detecting charged particles in a wide range of current because of their negative temperature coefficient of resistance.
Recently, there is an increasing demand for detection of charged particles in a range of from a slight amount to a large amount with one detector. To this end, the secondary electron multipliers are required to have a wide range of proportionality of input current to output current. In general, it is said that in an output current taken out from the secondary electron multipliers of a channel type, which increases linearly with the input current, is about 10% of the current of an multiplier element, that is, the current flowing through the electron multiplier element. In order to increase the output current, it is required to increase the current flowing through the multiplier element. If the multiplier elements are so designed as to have a wide dynamic range, that is, to have a greatly low resistance, they are self-heated as a result of current flowing through it. Thus, the resistance of the element decreases because of the negative temperature coefficient of resistance (as temperature increases, resistance decreases), which in turn causes increase of heat generation, resulting in thermal running away of the elements. Thus, making it impossible to retain a high voltage applied.